It is known to dope the core of an optical fibre with a rare earth element in order to amplify an optical signal. For example, an erbium-doped core pumped with a suitable wavelength pump source (for example 532, 670, 807, 980, or 1490 nm) can be used as a travelling wave amplifier for optical signals in the 1550 nm telecommunications wavelength region.
When such fibres are supplied with a light source having a wavelength capable of bringing the rare-earth atoms to an excited energetic state or pumping band, the atoms spontaneously decay in a very short time to a laser emission state, and stay in that state for a relatively longer time. When a fibre having a high number of atoms in the excited state in the emission level is crossed by a luminous signal with a wavelength corresponding to such emission laser state, the signal causes a transition of the excited atoms to a lower level with a light emission having the same wavelength as the signal. A fibre of this kind can thus be used to amplify an optical signal.
It is also known to form fibre lasers from erbium-doped fibres by creating at least one Bragg grating in the fibre. Such Bragg gratings comprise a periodic spatial variation in refractive index. In one type of fibre laser, referred to as a distributed Bragg reflector (DBR) laser, two Bragg gratings form end reflectors at respective ends of a gain medium (e.g. erbium-doped silica). Since Bragg gratings only reflect within a narrow spectral band around a Bragg wavelength, the mode closest to the Bragg wavelength will experience stronger feedback than other modes.
Another type of fibre laser is a distributed feedback (DFB) fibre laser in which a single Bragg grating is formed in the gain medium. A periodic spatial variation in the refractive index of the gain medium (e.g. erbium-doped silica) causes an optical wave travelling in a forward direction to be progressively reflected by the grating into a wave travelling in a backward direction, and vice versa.
DFB fibre lasers tend to have a better single-frequency stability than DBR fibre lasers. This is because the laser wavelength of a DFB fibre laser is determined by the Bragg wavelength of a single grating. Also, single longitudinal mode operation is promoted because the threshold gain in a DFB fibre laser increases with wavelength away from the Bragg wavelength.
Erbium-doped fibre lasers offer several key advantages that make them attractive for use in remote sensing applications. In particular, their ability to sense changes in strain with very high resolution has opened up a number of novel sensor possibilities. For remote sensing applications of DFB fibre lasers, reflection sensitivity and noise performance are important considerations. Both of these characteristics are dependent on the product of K and L of a fibre Bragg grating, where K is the coupling coefficient of the grating and L is the length of the grating. Reflection sensitivity and laser noise performance are expected to improve with increasing KL value of the grating, up to a limit defined by an onset of deleterious changes in complex susceptibility. In order to promote single longitudinal mode operation of a DFB fibre laser, the fundamental longitudinal mode ideally has a lower threshold gain than higher order longitudinal modes. In other words, there is a threshold margin between the fundamental and the higher order modes. However, for practical gratings it is expected that grating phase imperfections and intracavity loss generally result in a reduced threshold margin between the fundamental and higher order modes. It is therefore desirable to improve the threshold margin for high-KL DFB fibre lasers.
The intensity of a longitudinal fundamental mode of a laser cavity is at a maximum in the centre of the phase shift of the host Bragg grating, while higher order modes have maxima which are located off-centre from the phase shift towards the edges of the grating. For this reason, non-fundamental modes are also referred to as “side modes”. One known method of improving threshold margin comprises suppressing side modes by apodizing the fibre Bragg grating so that non-fundamental modes undergo less Bragg reflection than the fundamental mode.